Phase and amplitude responsive control system



Oct. 31, 1950 A. M. GRASS PHASE AND AMPLITUDE RESPONSIVE CONTROL SYSTEM Filed June 16, 1944 INVENTOR.

ALBERT M GRA SS Patented Oct. 31, 1950 PHASE AND AMPLITUDE RESPONSIVE CONTROL SYSTEM Albert M. Grass, Quincy, Mass., .assignor, by mesne assignments, to the. United States of America as represented by the Secretary of War Application June 16, 1944, Serial No. 540,602

The-present invention relates to a motor control system, and it relates more particularly to a vacuum tube driving circuit for controlling the operation and direction of rotation of A. C. or D3 0. motors. An example of an application of thepresent invention would be in a servo mechanism, wherein a sinusoidal control voltage, which might vary in amplitude from a maximum, through zero, to a maximum out-of-phase relation, as determined for instance by a director or data transmitter or by manual means, would be impressed on the vacuum tube apparatus of the present invention, the output of which would be applied toa servornotor or data receiver, which may-take the form of a two-phase A. C. motor, or a D. 0. motor. The control voltage applied to the apparatus of the present invention may constitute an automaticcondition-responsive error voltage, the amplitude or phase relation ofwhich may; be controlled by the relative position of a load orldata receiver with respect to director or data transmitter, or it may be manually controlled." v t f 'I -leretofore, motors controlled by vacuum tube circuits were subjected to currents having harmonics'or D. C. componentstending to generate lieatin the motor, and sometimes output transformers were resorted to in order to eliminate the D. C. components. One of the objects of the present invention is to provide a motor-control system'which will overcome these disadvantages, andwhich will be compact, economical and efficient in ,operation.

li Referring now to thedra wings, wherein like refe rence characters indicate like parts,

:11represents a circuit diagram of a control system, for a two-phase A. 0. motor, constltuting an illustrative embodiment of the presenti'nventionj V :-Fig. 2 is a diagram showing the manner in which the input A. C. control voltage may vary in amplitude and in phase relation; and also showing for the system of Fig. 1 the relation between the output A. C. current applied .to the motor winding (which is similar to the inputcontrol voltage) and the sine wave applied to the cross-phase motor winding;

Fig. 3 is a diagram showing the relationship of thefinput voltage and the output current curves, the latter being first shown in its component portions as they are passed through the vacuum tubes;

Fig. 4 represents a circuit diagram of another embodiment of the present invention, also intended for controlling a two-phMeAC. motor,

4 Claims. (01. 318-207) wherein the tubes are of the permanently biased negative cutoff type which require no driving power since the control grids never become posipressed across the primary winding II of an inputcontrol voltage transformer I2 which carries two secondary windings, l3 and I4. This control voltage source In is fed from the A. C. power input and its output can'be varied in amplitude either in phase or out-of-phase with the A. C. power input. Four vacuum tubes l5, l6, l1 and 18, are provided, I5 and I6 of which are controlled by secondary winding [3, and I1 and I8 by secondary winding I4. These vacuum tubes referably arenon-conducting when their control grids are at zero A. C. potential. This may be achieved by selecting tubes having the proper characteristics for such action, or by inserting resistance in the cathode circuit, or by other means. Considering now vacuum tubes l5 and i6 associated with secondary winding l3, the control grids l9 and 20 thereof are connected to one end of secondary winding [3 through lead 2|. Cathode 22 of vacuumtube I5 is connected at junction 23 with lead 24 which extends from the other end of secondary winding 13 to the centertap point 25 of the secondary winding 26 of a power input transformer 21. Cathode 28 of vacuum tube It is also connected to line 24 at the junction 29. The anodes 30 and 3| of vacuum tubes 15 and I6, respectively, are separately conends of a second secondary winding 34 in the power input transformer '21.

Vacuum tubes l1 and I8, associated with the secondary winding l4, are connected in similar butopposite fashion. Thus control grids 35 and 36 of these vacuum tubes are connected to one end of the transformer secondary winding 14 through lead 31. Cathode 38 of vacuum tube I1 is connected at junction 39 with lead 40 which extends from the other end of secondary winding l 4 to the center-tap point 4| of the secondary winding 34 of power input transformer 21. Similarly, cathode 42 of vacuum tube I8 is also connected to lead 40 at junction 43. The anodes 44 and 45 of vacuum tubes l1 and 18, respectively, are separately connected through leads 4-5 and 41 to the opposite ends of the secondary winding 26 leads 24 and 40 at junctions and 5|, respectively. The other orcross-phase winding 52 of the servomotor, represented in phantom fashion by dotted lines in Fig. l, is driven from an A. C. source of constant magnitude.

The operation of the system illustrated in 'Fig. l is as follows. The input A. C. control voltage may vary in amplitude from a maximum, as represented by sine curve 54 in Fig. 2, to-alesser amplitude, as represented lby-dashed curve 55, through zero, to a maximum opposite phase relation, as represented by dashed curve 56. It will be assumed that the two secondary windings 13 and I4 of the input control Voltage transformer l2 are arranged so that their upper ends are of like polarity, and that the two secondary windings 26 and 34 .of the powerinput transformer 21,

are arranged so that their upper ends are of opposite polarity. The A. C. power input applied to the primary winding of the power transformer 21 should be of the same-frequency as the applied control voltage, and either in or 180 out of phase therewith. Assuming that the input-control voltage appliedto the transformer primary winding H is of the magnitude and phaserelation shown by curve 54, then during the first or positive half of the cycle, when the input control voltage signal makesthe upper end of secondary winding I 3 most positive, thelower end of the second secondary winding 14 is relatively nega' tive, and the instantaneous conditions then obtaining are as follows. Control grids l9 and of vacuum tubes [5 and IG-areeach positive. Since anode of tube I5 is connected with the then positive end-of secondary .winding 34 and is. therefore itself positive, current vwill flow through tube l5. Tube l6, however, will not conduct since its anode 3| is connected'to the then negative end of secondary winding 34. At this same instant; the lower end of secondary winding I4 is of negative polarity and the upper end is of positive polarity making the control-grids 35 and 36 of vacuum tubes 11 and [8 negative with respect to the cathode elements 38 and 42. Therefore, neither vacuum tube I! or l8will conduct during this positive half'of the input cycle. It will thus be seenthat during the first or positive half of the input cycle of the fourvacuum tubes provided, only vacuum tube 15 will conduct, the flow being through'servomotor winding;49 around the circuit including-cathode. 22, anode 30, lead 32, secondary winding 34, to center tap point 4| and thence through lead -40-, serv omotor winding 49 and lead :24, back to the cathode 22. The current passedthrough vacuum tube l5 dur ing-the positive half of the input control voltage cycle is pulsating, asrepresented by'curve 51' in Fig. 3.

During the second or negative. half of the input control voltage cycle, all the" relative polarities mentioned a bovearereversed, so that now controlgrids 35 and 36-01 vacuum tubes IT and I8 become positive with respect to cathodes 38 and 42. Since anode 45.0f vacuum tube I8 is connected with the now positive end of secondary winding 26 and is therefore itself positive, current will flow through .tube 1%. Tube ll, however, will not conduct since its anode 44 is connected to the then negative end of secondary winding 26. At this same instant the lower end of secondary Winding I3 is positive and the upper end of that winding is negative, making the control grid elements [9 and 20 of vacuum tubes I 5 and [6 negative. Therefore, neither vacuum tube l5 or [B will conduct during this negative half of the input cycle. It will thus be seen that during the second or negative half .of the input cycle, of four vacuum tubes provided; only va'cuumtube 18 will conduct, the flow being through servomotor winding 49 around the circuit including cathode 42, anode 45, lead 41, secondary winding 26, to center tap point 25, and thence through lead 24, servomotor winding 49 and lead 40, back to the cathode 42. The currentpassed through vacuum tube I8' duringthe negative half of the input control cycle isalsopulsating, as represented by curve 58 in Fig. 3.

The combined effect of the positive current pulses 51 passed through tube I5 during the positive half of each input cycle, and the negative pulses 58 passed, through vacuum ,tube lt during the negative half of. each input .cycle p'rovidesa composite, current wave. 59 across, the servomotor winding 49, which causes the servomotor. to rotate inonedirection. It will be seen from- Fig. 3 that this resultantmotor current curve.5!l

control voltage curve-54.; V

i If theamplitude of thepontrolvoltage waveis reduced,.as forinstance .to ,;values represented by the dashed. curVe. 55.-of-, Fig 2., theamplitude of thepositive and negativepulses 51 and 5 8 (see is similar to and in phase withIthe applied input Fig.-. 3) would be. correspondingly reduced,

represented by. pulses .6] .and 62, respectively, to produce a composite current wave 63 through servomotor Winding- 4 9... lesser. amplitude, but,

of the same phase, as that represented'by' curve 59. Thus, change in the .amplfitudeof the input control voltage merely. res'ultsin a co rresponde ing change in amplitude of the components'offtlie output wave Whichcombine to provide thealternating current throughthe servomotor windings If now the amplitude of the input control-yolkages applied to the transformerprimary winding H shifts through zeroand changestoan opposite phase relationship, as represented by dashed curve 56 of.Figs. l and 2, then duringth'e first or.nega-.-

secondary windingl '3.'is. of negative polarity,.mak-- ing the control grid elements. I 9. and,20.of vacuum.

tubes l5 and, I 5,.ne'gative. Therefore, neither vacuum tube v I 5. or 1.62 will conduct. during, this f rst or negative half ofthefinput control .voltage.

cycle. It will thusbeseenthatduringthis first or negative half of tlihihput c ycle, ,of, the four vacuum tubes provided'j only vacuum tube ll conducts. current,.the iiow then being through the servomotor winding '49; around. the circuit including cathode 38, anode 44, lead, secondarywinding 26, to center tap point 25', and1thence through lead. 24', servomotor winding 43 'and'lead back to thecathode 38. The current passed.

through vacuumtube l7 during this first or nest.-

ing 34, andis therefore itself positive, current.

will flowthrough tube I6. Tube I5, however, will not conductsinceits anode is connected. to

the thenmegative end of secondary winding .34.

At the same instant, the lower end of secondary wintiingr I 4 is of negative polarity and the upper I end. of that winding is of. positive polarity, making the control grid elements and 36 of vacue.

um tubes-11: and I8"negative with respect to the cathodeuelementsl38 .and 42. Therefore, neither, vacuumrtube I1I .or allitlwill conduct during. this second for positive .half... of the input cycle. It

willthus be seen that during the second or positive.-'half;. ofrther input control voltage cycle, of the,;four .avacuum tubes provided, only vacuum tube I6.will conduct current, .the flow being through,servomotor-winding 49,. around the circuit includingcathode .28, anode 3I,.lead 33, secondary-. .Winding -j3 4,- to center, tap point M, and thence, through. lead 40, servomotor winding 49 and lea,d .2, 4, back to the cathode 28., The current passed throughvacuum tube I6 during the second,

or; positive half of the input control voltagecycle isqalso pulsa-ting,;opposite in phase relation to the pulsesij. illustrated in Fig.3. 1

passedthrough tube' ,I,'I during the negative half,

if-each inputscycl ithe p t p s passed hr u h.v uumtu el 'idurineith p s e halfv ies ;.innutwc c.p ov de a c p s te t.

across the servomotor winding 49, which is ut ofphase with.l the composite wave j 59 T his causes the servomotor siteydirection to that produced dic ate in ans bythe; currentcurye 59. It. will thus .be seen that. theresultant motor current curve is similar in. form but, liiOf-out-of-phase with respect to the,

applied input controlvoltage curve 56.

,-;Therefore, in this first embodiment, when the control voltage is v of the phase relation represented by, curve ,-54;in Fig. 2, tubes I5 and I8 alternately conduct current during opposite halves of theinputcontrolvoltage cycle. However, should the control voltagebecome of opposit phase re-, lation as represented by dashed curve 56 in Fig; .l...;then;l tub s .IB. and-; l1 will alternetelylconduc current during, opposite halves of theiinput controivolta cyc e-nx 1 5r :desired; the .uhereinabove-discussed variations; in; magnitude onpolarity of the A. C: "control..volt age may be. achieved by manually varyingixcircuita resistance, inductance or capacitance, ,orJthe position of the control voltage transformer primary relativetothe secondaries.

=-I'n thembo'd-iment illustrated in Fig. 4, four vacuum tubes 65,:69, 61; and-68'are provided. th tiibes operating en the-lowerportion of their characteristic curves, so that thegrids never be-' These-tubes, therefore require no driving'po'wer. TheA. CJcontrol voltage in-- put signal'isappl'ied across terminals B9and 10,

come positive through blocking condensers II and I2, to j uncl5 menace I3, whilecontrol grids I9 and-80 of vacuum tubes Gland 6B are connected by-lead 8I to junction I4.

A resistance network comprising two series re'- sistors 82 and 83, meeting at junction 84, is connected across-junctions I3 and I4. Cathodes 85 and 9| of vacuum tubes 65 and 66 are respective-"u 1y connected at junctions 8B and 92 with lead 81 extending to the center-tap point 88 of a secondary winding 89 on the power input trans former: 90; Similarly, cathodes 93 and 98 of vacuum tubes 61 and 68 are respectively con-- nected at junctions 94 and 99 with lead 95 extending to the center-tap point 96 of a secondi secondary winding 91 in the powerinputtran'sformer 90. A pair of series cathode resistors I00. and NI, meeting at junction I02, are connected across junctionsBB and 94. Thedriving voltage, for the servomotor winding 49, which is connected across junctions 92-and 99, is developed across cathode resistors I00 and IOI. The grid bias may. be obtained from a C battery I03, whose positive terminal may be connected to .junction I02 and whose negative terminal may'be connected to junction 84 intermediate the series resistors-.82 and 83. i

In order to overcome the degenerative eifect. of this cathode follower tube arrangement, con.-. densers I0 I-and I05 mayube. connected. across' the cathode junctions 86 and '94 on the one hand, and resistor intermediate junctionsv I06 and I01 on the other hand. The anodes I09 and I09::of vacuum tubes65 and B6 are separately connected, through leads H0 and III to the OPpOSiteJGIIdS- of the power input transformer'secondary winding 91,. While anodes I I2 and [I3 of vacuum tubes 91 and 69 areseparately connected through leads:- II4 and H5 to the opposite ends of the power. input transformer secondary winding 89. The A. C. motor winding may be connected across junctions Hand 99. The A. C. power input is. applied to the primary winding II6 ofthe power transformer 90.

Theyoperation of the motor control. system" illustrated in Fig. 4 is as follows; Assumingthat the input A. C. control voltage applied across, terminals69 and I0 is represented by the voltage curve 54, and that in this embodiment (as in the one illustrated in Fig. 1) the two power input transformer secondaryiwindings 89 and 91-are arranged so that their upper ends are of oppo site polarity, and that the A. C. power input applied to the transformer primary winding H6 is of the sam frequency as the applied control voltage and in phase therewith, then during the first or positive half 'of the cycle, with the con trol grids I5 and I6 changing in apositive-g'oing direction and control grids 19 and 80 changing in a negative-going direction, the instantaneous conditions then obtaining are as follows. Con' trol grids I5 and I6 of vacuum tubes 65*and66 are each positive relative to cathodes 85 and 9I in view of the voltage drops aeross'resistors 82 and I00. Sinceanode I08jof vacuum tube 65is connected with the then positive end of second ary winding 91, and is therefore itself positive} current will flow through tube' 65, aroundth'e circuit including cathode 85, anode I09, lead H0," secondary winding 91, center-tap point 9:6,,flead 95, motor winding 49, lead 81, and thenback 'to' cathode 85. Tube 66,-however, willnot conductg since its anode I09 is connected to the opposite then negative end of secondary windingii ll At" this same instant, the control grids 'I9and 80 of vacuum tubes 6] and 68 are negative relative to? their respective cathodes 93 and 98, so that neither-oneiof these twoitubes willconductxdur= ing this first or positive half of the input control become positivewith respectto cathodes 93 and 98. Since anode II3 of vacuum tube is connected with the then positive end of secondary Winding 89, and is thereforeiitself positive, current willflow through vacuum tube. 66,.around the circuit including cathode 98, anodeil I3, lead II5; secondary winding 89, center. tap point 88, lead 81, motor winding 49, and thence back to cathode 98. Vacuum tube.61, however, will not conduct since its anode H2 is connected to the then:negative end of secondary winding 99. At thistsame. instant, the control grids'15. and 15 of vacuum tubes 65 and 66 are negativetrelative to: their respective cathodes 85 and 9|, so that neither one of these two tubes will conduct during this-second or negative half of the input cycle.

It will therefor be understood that the combined effect of the positive current pulses passed through vacuum tube 65 during the positive half of each input cycle, and the negative current pulses passed through vacuum tube 68 during the negative half of each input cycle, produces av composite A. C. current wave across the motor winding-49, causing the motor to rotate in one direction. In the. event that th control voltage should shiftathrough zero and change. to'a.l'80? out of-phase relation; as represented by dashed curve 59. inFig. 4, the relationship of input Sig:-

nal: polarities to transformer polarities reverses,

so that now during the. first or negative half of each input cycle-it will be'seen that only vacuum.

tube 61 conducts, and during the second or positiveha-lf of each input cycle, only vacuumtube 66 conducts. The combined effect .of these alternating pulses is to produce a composite A. C..

current waveacross'motor winding 49: which is 180. .out-of-phase with theA. C. currentwave. produced when the...control voltage curve. is represented by 54, so'that the motorwillnowrotate.

in. the reverse .direction;

The embodiment. shown in Fig. illustrates: how an A. C. control voltage, amplified by 'thecircuit of the present invention,. may-be applied", to. the winding of. a D. C. motor to control not only the extent but also thedirection'Qfrotation of the motor. In this embodiment; the A. C.

control voltage input signal is applied across the terminals of the primary winding I 2010f anxinput'. control voltage-transformer I2I, which carries? two' center-tapped secondary windings I22.land- I23. Four vacuum tubes I24, I25, I26-and I2FIi are provided, the first two of which are controlled.

by secondary Winding I22, and the last two by: secondary winding I23. Thus, COIItI'OI gIldSJ28;

and- I29-of vacuum tubes I24 and I25 are con.- nected to the opposite ends-of secondary wind.- ing-I22, and'control grids I30 and I3'I ofvacuum point I31 of secondary winding I22 toa centertap point I38 of a secondary winding I39 in the power input transformer I 49. Similarhncathodes HI and I42 of vacuum-tubes I26 and. I21: are respectivelyconnected atjunctions I 43andi I44 with lead. I45 extending fromcenter-tap 83 point I 4.6 of secondary winding. I23- ta a center tap'point. I41 of a second. secondary winding;I48-' in the power transformer I40. Anodes I49 and? I50 of vacuum tubes I24and I are respectively connected withthe-opposite ends of'the power; transformer'secondary winding I48, andanodes. I5I and I52 of vacuum tubes I26 and. I21 are respectively connected with the opposite ends of the. other power transformer'secondary winding; I39; The A. C. power input is fed into theprie' mary winding I53 of:the power transformer I40;

The winding of the D. C. motor'l54 which is;

controlled. by the embodiment illustratedin Fig.

This motor may be as Theoperation of the motor 'control system il lustrated in .Fig'. 5 is as follows.' Assuming that;

the input A. C. control voltage applied across .pri'

mary winding I20 is represented by voltage curve.

54-, and that in this embodiment (as in thezone illustrated in Fig. 1) the two secondary windings I22 and I23 of the input: control voltage.- transformer I2I are arranged so that their up-- per ends are of like polarity, and that the two.

secondary windings I48 and I39 of the power input transformer I40 are arranged so that: their upper ends are of opposite polarity, and further assuming that the A. C. power inputapplied to the power transformer primary winding I53 is.

of the same frequency and in'phase with the applied'control voltage, then during the first or positive half of the input cycle the instantaneous.

conditions are as follows. Control grids I28 and I3I of vacuum tubes I24'and I21 are each-posi tive with respect to their respective cathodes I32 and I42. Since anode I49 of vacuum tube- I24 isthen positive-and anode I52'of vacuum'tube I21 is then negative, only tube I 24 will conduct during this first or positive half'of the cycle. The negative polarity of control grids I29 and I relative to cathodes I 33 and I M of vacuum tubes I25 and I26 prevents any conduction by these tubes during this first half of the cycle. The current passed through vacuum tube I24 follows along the circuit including cathode I 32, anode I49, secondary winding I48, center-tap point I41, lead I45, junction I44, D. C. motor winding I 54, junction I35,-lead I36, junction I 34', and thence back to cathode I32.

During the second or negative half of the inputcycle, all the relative polarities are reversed, so that now control grids I29 and I30 of vacuum tubes I25 and I26 are positive with respect to their cathodes I33 and I4-I. vacuum tube I25 is then positive, and anode I5I of vacuum tube I26 is then negative, only-vacuum tube- I25 will conduct during this second or negative half of the cycle. The. negative polarity of control'grids I28 and I3I relative toscathodes;

I32 and..l42 of vacuum tubes I24 and l21=pree vent any conduction through-these tubes :during:

this second or negative half of thecycle. The current passed through vacuum tube I25 followsialong the circuit including cathode I33, an-- ode I50, secondary winding I 48, centertap point. I41, lead I45, junction I44, D. C. motor winding; I54, junction I and thence back to cathode I33. It will be observed that the current passed through vacuum tube I25 and D. C. motorwind+ ing I54 during this second or negative half of the input cycle is in the samedirection as that passed through vacuum tube I24 and said D. 0,.

Because 'of'theunidirection aspect of the resultantcurrent flowing through. the D. C. motor winding, the motorwill tend to rotate in one directioni However, should the control voltage shift through 'zero' to a 180 out-of-phase relati'omlas represented'by dashed curve 56 of Fig. 5,

I then the relationship of polarities of the windings of the input control voltage transformer I2I to thepolarities of the windings of the power input transformer I40 reverses, so that nowduring the first 'orneg'ative halfof each input cycle, it will be seen that only vacuum tube I26 will conduct, and duringthe second orpositive half of each input cyclei only vacuum tube I21 will conduct.

Here again, because of thephase opposition of vacuum tubes I26 and I21, they operate in push pull fashion to maintain unidirectional current flow through the D. C. motor winding I54 during alternate halves of the input cycle, the cur- "rent flow through the motor winding inthis case,

however, being in a direction opposite to that resulting when the control voltage curve was represented by 54 with vacuum tubes I24 and I25 conducting. This reversal in direction of current flow through the D. C. motor winding causes the motor to rotate in the opposite direction.

If desired, the circuit illustrated in Fig. 5 may be adapted to employ negative grid cutoif vacuum tubes in a manner similar to that shown in Fig. 4.

The motor control systems hereinabove discussed and forming the subject matter of the present invention are also applicable for use with grid-controlled gas discharge tubes, an example of which is marketed under the trade name Thyratron.

In view of the present trend of motor manufacturers to increase the impedance of the motors, the output of the present invention may be applied directly to the'motor windings without the use of any matching transformers.

Having thus described the invention, what is hereby claimed as new and desired to be protected by Letters Patent is:

l. A control system comprising a. power transformer having a primary adapted to be supplied with alternating currentand having two centertapped secondaries, two pairs of electron discharge tubes, each such tube having a cathode, control grid and anode, means for connecting an anode of each tube to an end terminal of said secondaries, one pair of tubes being connected to one secondary, the other pair of tubes being connected to the other secondary, an alternating current motor having one winding through which the magnitude and polarity of current is to be controlled, means for connecting the cathodes of one pair of tubes to one motor winding terminal, means for connecting the cathodes of the other pair of tubes to the remaining motor winding terminal, a center-tap connection from said one secondary going to the cathodes of the other pair of tubes, a center-tap connection from the other secondary going to the cathodes of the one pair of tubes, a source of control voltage of the same frequency as the power transformer current supply, means for varying the amplitude of and "changing the phase of said control voltage jthrough 180, and means for applying said control voltage to the control grids and cathodes of all tubes, said means applying control voltages of the same phase to the one pair of tubes and of opposite phase to the other pair of tubes.

T 2; In a motor control system including a source of A. C. power, an A. C. control voltage source operating at the same frequency as said power source and adapted tovary'from an in-phase to a 180 out-of-phase relation relative thereto and an A. C. motor comprising at least two windings to be fed'in phase'quadrature, one of said windings connected to saidsource of A. 0., a driving circuit fo-rsaid A. C. motor including two pairs of vacuum tubes, each vacuum tube having a cathode, a control grid and an anode, a power insecondary, the cathodes of the first pair of vacuum tubes being connected to the center-tap point of the last-mentioned transformer secondary, and the cathodes of the second pair of vacuum tubes being connected to the center-tap point of the other transformer secondary, the control grids of each pair of said vacuum tubes being coupled together, and to the control voltage source, and the A. C, motor having the other of said windings connected across the cathodes of said pairs of vacuum tubes, saidvacuum tubes being so arranged that when the A. C. control voltage is in phase with the A. C. power source, one vacuum tube of each of the aforesaid pairs of vacuum tubes alternately conduct on opposite halves of the cycle thereby tending to rotate said motor in one direction, and when the A. C. control voltage is 180 out-of-phase with the A. C. power source, the other vacuum tube of each of the aforesaid pairs of vacuum tubes alternately conduct on opposite halves of the cycle, thereby tending to rotate said motor in the opposite direction.

3. In a motor control system including a source of A. C. power, an A. C. control voltage source operating at the same frequency as said power source and adapted to vary from an in-phase to a 180 out-of-phase relation relative thereto and an A. C. motor, a driving circuit for said A. C. motor including two pairs of vacuum tubes, each vacuum tube having a cathode, a control grid and an anode, a power input transformer having vacuum tubes being connected to the opposite end terminals of one of said transformer secondaries and the anodes of the second pair of vacuum tubes being connected to the opposite end terminals of the other transformer secondary, the cathodes of the first pair of tubes being connected to the center-tap point of the lastmentioned transformer secondary, and the oathodes of the second pair of tubes being connected to the center-tap point of the other transformer secondary, a control voltage transformer having a primary and two secondaries, the control voltage source being impressed across the control voltage transformer primary, the control grids of S id fi t pair of Vacuum tubes being connected motor being connected across the cathodes of said pairs of vacuum tubes.

4. In a motor control system including a source of A. C. power,an A. C. control voltage source operating at the same frequency as said power source and adapted to vary from an in-phase to a 180 out-of-phase relation relative thereto, and amotor, a driving circuit for said motor including two pairs of vacuum tubes, each vacuum tube having a cathode, a control grid and an anode, a power input transformer having a primary and two center-tapped secondaries, the source of A. C.

power being impressed across said primary, the anodes of the first pair of vacuum tubes being connected to the opposite end terminals of one of said transformer secondaries and the anodes of the second pair of vacuum tubes being connected to the opposite end terminals of the other transformer secondary, the cathodes of the first '12 pair of vacuum, tubes being-connected to thementer-tap point of the last-mentioned;transformer secondary and; the cathodes of I the: second .pair of vacuum tubes being'lconnected to the centertap ;-point of the wother transformer secondary, the control-grids of-each pair of; tubes being connected together and .the control voltage source 7 being-applied across sa-id-;grids-.through a pairaof blocking condensers, -a resistance networkin- -clud-ing 1 two series resistors shunted across "the control grids of the pairs of vacuum tubes, 'gridbiasing means intermediatetheoathodes andi-the control grids of eachpair, of vacuum tubes a-pair of I cathode resistors -connected= across the ::cath odes of the -pairs 10f vacuumtubeseand a-pa'ircof condensers connected between the cathodesaand an inter-mediate point-of each series'resistor, the

f motor being connected across the-cathodes; :ofeaid pairsof' vacuum tubes.

:AIJBERT M. GRASS.

-R'EFERENGES GITED "The'following references are of record im the file of this patent:

UNITED "STATES PATENTS Number Name Date 1 958245 Mittag et a1 .'May 8, 1934 2,218,477 Parker Oct. 215,:1-940 

